Sound identification based on auditory is one of the most ancient signal identification methods, and is still wildly used in many fields nowadays. Auscultating the sounds of human heart and lung by means of a stethoscope for medical diagnosis is an application example. This auscultation method has the advantages of simple and easy to apply, therefore, many research organizations and individuals engaged in research and development of auscultation equipment. For example, Chinese patent numbered 2009200623488, “Active noise reduction electronic stethoscope”, can actively remove environment noise, to improve the accuracy of stethoscope and the effect of auscultation; for another example, Chinese patent numbered 2011101883094, “A wireless stethoscope”, may realize remote auscultation by means of a radio signal transmitting and receiving device. All of these studies, the signal source are limited in the audible frequency range; that is, the signal source is audio signal conducted by vibration with a frequency limited from 20 Hz to 20 KHz, audible by human ear.
However, with the development of science and technology, there is a large number of information in the inaudible or non-sound signals that a human ear cannot hear. For example, the lunar surface topography fluctuation, the yearly precipitation change in a region, the human brain waves fluctuations, the voltage change of an electronic circuit, stock price fluctuations, or the statistical results of a physical phenomenon, etc., that can be recorded and expressed in a format of waveform. Those waveforms either fall outside the frequency range of an audible signal, or are transferring in a way of no-vibration, therefore cannot be heard by human ears and are inaudible signals.